Effect Of Polluted Sediment On Water Quality In The Seto Inland Sea

The Seto Inland Sea, which is surrounded by three major islands of Japan, has about 400 km in width and the average depth of about 30 meters. An exchange of the inland sea water with the outside ocean water occurs only through two channels (the Kii and Bungo) and Kanmon Strait. This sea is typical of the large semiclosed inland sea in the world. The surrounding land has attracted large population and enormous build-up of industrial, resulting in large increase in effluent of pollutant into the sea. Most of pollutants derived from the surrounding land deposit on the bottom floor in the inland sea for lack of flushing tidal power which brings them to the outside ocean. The deposited pollutants decompose in the bottom sediment and consequently a large amount of nutrients and heavy metals such as Mn and Fe are released from the bottom sediment. The nutrients and heavy metals-rich bottom water has brought about serious water pollution problems such as the outbreaks of red tide. In order to know the extent of pollution caused by the bottom sediment, the vertical distributions of water quality (nutrients, dissolved oxygen, etc. ) and their seasonal variation were investigated at points of more than 200 sampling stations in the Seto Inland Sea (Fig. 1). In summer, it is found that all stations show a similar behavior on the vertical distributions of water quality. The representative example of the vertical distributions of the POL+-P, TIN, dissolved manganese, dissolved oxygen and water temperature are shown in Fig. 2. A thermocline is found to cause at a depth around 10-15 m. The thermocline inhibits the vertical mixing between surface and bottom water, and as a result leads to the depletion of dissolved oxygen in the bottom layer. The nutrients released from the bottom sediment are early constant above the thermocline, whereas these concentrations below the thermocline increase with depth. It is also found that Mn has the same behavior as the nutrients. From the results obtained, as a rough approximation the net concentrations of POL+-P and TIN can be obtained by subtracting the constant concentrations above the thermocline from each measured value. The fluxes of POL+-P and TIN released from the bottom sediment were calculated by using the net concentrations at each station, respectively. In winter, on the other hand, the thermocline disappears over the whole area of the Seto Inland Sea, by which the strong vertical mixing occurs. The nutrients are distributed homogeneously from the surface to the bottom, which implies that a nutrient-rich water layer disappears, though the nutrients continue to be released from the bottom sediment during this season. Here it should be noted that their vertical distributions are much different between in winter and summer. Then we cannot apply the same procedure described above to getting the fluxes of the nutrients released from the bottom sediment using the obtained distributions. Hence another experiment was conducted on an oxygen consumption rate of sediment by using a bell-jar method to estimate fluxes of the nutrients, where the bell-jar method is developed to obtain more accurate data compared to the conventional ones as shown in Fig. 3. The oxygen consumption rate thus obtained is converted into produce rates of POL+-P and TIN by using the relation between the Apparent Oxygen Utilization and POL+-P/ TIN concentrations. And then the fluxes of PO4-P and TIN are estimated from their produce rates. From the fluxes obtained both in summer and winter, the mual fluxes of POL+-P and TIN in the Seto Inland Sea are estimated and are shown in Table 1. Their annual fluxes are considerable large as expected and become more than a half of those mual fliutes discharged from the surrounding land. It is therefore suggested that it is necessary to develop a new method for removal of the polluted sediment in the global point of view in order to protect the marine environment of the Set0 Inland Sea.